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Particle Zoo and
Feynman Diagrams
Cristina Lazzeroni
Professor in Particle Physics
STFC Public Engagement Fellow
ASE Conference 2016
University of Birmingham
Muirhead 118
7th January 2016, 10:00-12:00
Particles accelerated to speed of light :
E=
2
mc
Energy and mass are related, in general
E2 = p2 + m02 , c=1
(Invariant mass)2 = E2 – (px2+ py2 +pz2)
Protons smashing together can produce all sorts
of particles, seen in the earliest moments of the universe
The Modern Picture of Protons and Neutrons
In 1969, an experiment at SLAC
using a 2-mile long 20 billion eV
electron accelerator showed that
protons have structure  “quarks”
• Protons and neutrons made from
Up (u) and Down (d) quarks.
• u-quarks have +2/3 of electron
charge, d-quarks have –1/3
Proton
Neutron
Discovery of Quarks
γ carries momentum
Wave-function oscillates rapidly in space and time ⇒ probes short
distances and short time.
Rutherford Scattering
Excited states
E
!
Elastic scattering from
quarks in proton
λ<< size of proton
A Nice Happy
Family of Particles
Up quark (u)
Down quark (d)
Electron (e-)
Mass ~ 0.003
~ 0.006
~0.0005
(relative to the mass of a single proton)
Electron
neutrino (νe)
~ 10-8 ?
Everything around us (the whole Periodic Table) is
made of up quarks, down quarks and electrons.
But the masses don’t add up….
… for some reason, there is more …
leptons
quarks
Nature supplies us with a copy of the family but heavier …
… and another copy of the family but even heavier …
charm
up
down
strange
e
µ
νe
νµ
beauty
τ
ντ
The Top Quark
… and that is
where it seems
to stop
Weighs about the same
as a gold nucleus!
Top has no time
to hadronize
Antimatter
1928: Paul Dirac put together Relativity with
Quantum Mechanics into the theory of the
electron …
it also predicted anti-electrons (`positrons’)
Every fundamental fermion particle has an
antiparticle, with the same mass, but opposite charge.
electron
e-
e+
up quark
u+2/3
u-2/3
positron
up anti-quark
etc etc …
Antimatter
In unifying QM and Relativity, there is the need
of negative-energy particles: antimatter
Gravitational Force
Isaac Newton
(1642 - 1727)
Electromagnetic Force
James Clerk Maxwell
(1831 - 1879)
Size of atoms is set by strength of EM force
radioactive decays
How stars generate energy
Weak Force
Enrico Fermi
(1901 - 1954)
cay
e
d
n
o
r
t
u
ne
holding proton, nucleus
Strong Force
gluons
Size of nuclei is set by strength of strong force
LEPTONS
q
e–
ν1
µ–
Second
Generation ν2
τ–
Third
Generation ν3
First
Generation
Bosons:
Force carriers
integer charge
QUARKS
m/GeV
q
m/GeV
–1 0.0005 d –1/3
0.3
u +2/3
0.3
s
–1/3
0.5
0
≈0
–1
0.106
0
≈0
c +2/3
1.5
–1
1.77
b –1/3
4.5
0
≈0
+2/3
175
t
Leptons:
integer charge,
DON’T feel STRONG
Quarks:
fractional charge
feel ALL forces
Force
Boson(s)
m/GeV
EM (QED)
Photon γ
0
Weak
W± / Z
80 / 91
Strong
(QCD)
Gravity (?)
8 Gluons
g
Graviton?
0
0
Forces
Quantum Mechanically: Forces arise due to exchange of
VIRTUAL FIELD QUANTA
v
F
v
p
q1
q2
Force
Boson
Strength
Mass
(GeV/c2)
Strong
Gluon
g
1
Massless
Electromagnetic
Photon
γ
10-2
Massless
Weak
W and Z
W±, Z0
10-7
80, 91
Gravity
Graviton
?
10-39
Massless
Range of Forces
The range of a force is directly related to the mass of the
exchanged bosons
Force
Range (m)
Strong
Strong (Nuclear)
10-15
Electromagnetic
∞
Weak
Gravity
1 fm distance at speed c means 3 10-23 s
Uncertainty principle: E ~ 200 MeV i.e. pion
Predicted in 1935; discovered in cosmic ray
interactions in 1947
∞
10-18
∞
+
Time
=
Space
Space
Feynman diagrams
Time
FEYNMAN
DIAGRAM
Feynman devised a pictorial method
to evaluate probability of interaction
between fundamental particles
Virtual particles
Virtual Particles
Feynman diagram
a
c
b
d
time
space
space
“Time-ordered QM”
a
c
b
d
time
•Momentum conserved at vertices
•Energy not conserved at vertices
•Exchanged particle “on mass shell”
a
c
b
d
•Momentum AND energy conserved
at interaction vertices
•Exchanged particle “off mass shell”
VIRTUAL PARTICLE
•Can think of observable “on mass shell” particles as propagating waves
and unobservable virtual particles as normal modes between the source
particles
Conservations
Energy, momentum, charge
Baryon number, lepton number
Strangeness (and Colour) in Strong interactions
Trees
Penguins
The story of penguin diagrams
Particle Spin
Introduced by Pauli in 1924 as new quantum degree of
freedom which allowed formulation of Pauli exclusion
principle.
In 1925, it was suggested that it relates to self-rotation, but
heavily criticised… only useful as a picture.
In 1927 Pauli formulated theory of spin as a fully quantum
object (non-relativistic). In 1928 Dirac described the
relativistic electron as a spin object.
In 1940 Pauli proved the spin-statistic theorem: fermions
have half-integer spin and bosons have integer spin.
Particle Spin
Quantum mechanical, intrinsic angular momentum.
Particles are observed to possess angular momentum that
cannot be accounted for by orbital angular momentum.
Although the direction of its spin can be changed, an elementary
particle cannot be made to spin faster or slower.
Spin cannot be explained by postulating that they are made up of even
smaller particles rotating about a common centre of mass. Truly
intrinsic property.
Same behavior as angular momentum, quantised.
Integer and half-integer values: fermions (Pauli exclusion principle)
and bosons (Bose-Einstein condensation).
Hadrons = massive
Single free quarks are NEVER observed, but are always CONFINED in
bound states, called HADRONS. Macroscopically hadrons behave as
point-like COMPOSITE particles. Hadrons are of two types:
MESONS (qq)
•
Bound states of a QUARK and an ANTIQUARK
•
All have INTEGER spin, Bosons
( )
" # ! ud
( )
"+ ! u d
BARYONS (qqq)
•
Bound states of 3 QUARKS
•
All have HALF-INTEGER spin, Fermions
p ! (uud )
PLUS ANTIBARYONS
(q q q)
qq
qq
q
n ! (udd )
( )
p ! uu d
( )
n ! ud d
Isospin
Heisenberg in 1932 suggested that neutron and proton are
treated as different charge states of one particle, the
nucleon.
Idea originally introduced in nuclear physics to explain
observed symmetry between protons and neutrons: e.g.
mirror nuclei have similar strong interaction properties
A nucleon is given a number ISOSPIN I and then there
are 2 states with I3 with value +1/2 or -1/2:
I = 1/2 , I3 = +1/2, -1/2
Mesons were firstly postulated as the carrier of the strong force.
In 1947, the charged pion was discovered by a group of scientists
in Bristol, looking at photographic emulsions exposed to cosmic rays.
For ISOSPIN, one can put pions in a set of 3
They are formed by u and d quarks
Charge Q = I3 + B/2
B=baryon number
Relevance of ISOSPIN now understood as a consequence of
very similar masses of u and d quarks, and conservation of isospin
in strong interactions.
About at the same time, scientists from Manchester found
another new particle in interactions of cosmic rays in a cloud
chamber.
More and more were seen, and their rate of production in
pion-proton reaction was measured and their lifetimes were also
measured.
This new kind of particle was STRANGE in the sense that they
decay very very slowly but the reaction production proceed very
fast. Besides, they are always produced in at least 2 of them.
So they were called STRANGE particles !
Produced in pairs
Now understood in terms of s quark and the fact the strong
interaction conserve any quark type, so conserve also strangeness.
Q = I3 + (B+S)/2
B=baryon number
This is purely because s quark is not so much heavier
Now classify mesons and baryons on basis of I3 and B+S:
Quark Model
B+S
Same mass
Same jump
in mass
I3
Same mass
Same mass
Spin=0 mesons
Spin=3/2 baryons
Possible to extend to include charm and beauty but less
and less precise the more the quark is heavier
Higgs mechanism
Supersymmetry
Only a small fraction of the total is ordinary
matter that we know !
Resources:
Quark game
Top-trump card game
Plus various others from websites:
http://www.the-higgs-boson-and-beyond.org/
http://www.ep.ph.bham.ac.uk/DiscoveringParticles/
http://www.understanding-the-higgs-boson.org/
http://www.ep.ph.bham.ac.uk/user/lazzeroni/outreach/harry_card_game
Particle Zoo:
http://www.particlezoo.net/
Other Particle Physics Sessions:
Particle Zoo and Feynman Diagrams
Muirhead 118
7th January 2016, 10:00-12:00
ASE Frontier Science Lecture: The Mystery of Antimatter
Poynting Physics S02
7th January 2016, 14:30-15:30
Schools’ STEM exhibitions: HiSPARC project
Aston Webb Great Hall
8th January 2016, 9:00-12:30
Particle Teaching Resources: LHC Minerva
Learning Centre LG13
8th January 2016, 9:30-11:30
Particle World for Primary
Arts LR8
8th January 2016, 13:45-15:45
Please don’t forget to complete the
on-­‐line evalua7on for this session
h8p://bit.ly/AC2016FF